Process Integration/Layout of Drying Systems
The integration of a drying system into a process work flow for plastic processing can make a big difference in the success of your drying task, the ease of service and the need for additional equipment.
Equipment: Drying systems are typically made up of two major components - the dryer and the drying hopper.
The dryer typically provides heated, dry air to the hopper and is connected to utilities which depends on the dryer type. These may include:
- Electrical supply
- Gas supply
- Compressed air
The drying hopper is the vessel that holds your plastic material and it is connected to the dryer with hoses or tubing for the transfer of heated, dry air to the hopper for drying. The hopper is typically equipped with:
- A vacuum loader or vacuum receiver to fill the hopper
- A slide gate at the bottom to allow the material to be drawn out of the hopper to the process machine
- A port, to allow materials to be drained from the hopper for material changeover
- Insulation to maintain resin temperature
Some dryers combine these components into a single cabinet or onto a common stand or cart for easy installation and/or portability.
Equipment Location: There are several choices when it comes to dryer location.
- Machine mounted advantages include:
- No chance of material loss, contamination or exposure to ambient air that might cause moisture pickup
- Ideal for systems dedicated to a single material
- Off-machine location advantages include:
- No need to climb on ladder for hopper cleanout
- Easy access to loaders and dryer for maintenance
Portable Drying Systems: The ultimate in flexibility for small dryers.
- These compact systems usually integrate a dryer, hopper and loader on single cart with casters for easy transfer from one process machine to another. The cart size usually gets impractical with throughputs over 200 lb./hr.
Mezzanine Mounting: Can be utilized in different ways
- Dryers may be mounted on a mezzanine directly above the process machines they supply
- The mezzanine may be used as a location for the drying equipment that saves floor space and allows any dryer to supply dry material to a number of processing machines, below
- Ideal to minimize equipment on processing floor and for clean rooms
- Reduces initial cost of system and installation
- Drying hoppers mounted directly on the machine throat may allow gravity flow to the machine throat, eliminating the need for additional receivers or loaders – this does not work with batch-drying equipment
Central Drying System: Dries multiple materials with a single central dryer
Additional Equipment: Drying equipment location can affect the need for support equipment
Moving fresh material to the drying hopper (and in many cases, moving dried material away from the hopper) is a large factor when considering layout. Any equipment needs to be occasionally serviced, and loading equipment high atop a large drying hopper is no exception, so it should be taken into consideration. When transferring dried material, considerations should be made as to distance and how often the material will be moved and how long it will be at rest away from the drying hopper before it is consumed by the processing machine. As a result, purging the take-away material line of all dried material after each load should be considered.
For this, a purge valve at the base of the hopper is employed which allows conveying air to sweep the transfer line clean after the purge valve closes off material flow from the hopper. In many cases, using a separate or ‘borrowed’ supply of dry air for conveying is recommended, so that even during conveying, the material is in contact with only dry, not ambient air.
In some cases, typically for larger drying systems, additional equipment for filtering closed loop dry air or coolers for the drying air stream are required and their location close to the drying process can drive the location of the drying systems dryer and hopper.
In summary, the flow of fresh material from storage, to the drying system and finally to the processing machine(s) should be fully examined, along with the need for all additional pieces of support equipment before a layout decision is made for a drying system. In addition, service of all associated devices should be considered.